In a wireless communications system such as a mobile cellular communications system, a wireless local area network (WLAN), or a fixed wireless access (FWA) system, communications nodes such as a base station (BS, Base Station) or an access point (AP), a relay station (RS), and user equipment (UE) are generally capable of transmitting their own signals and receiving signals from other communications nodes. Because a radio signal is attenuated greatly in a radio channel, in comparison with a transmit signal of a local end, a signal coming from a communications peer becomes very weak when the signal arrives at a receive end. For example, a difference between transmit power and receive power of a communications node in a mobile cellular communications system may be up to 80 dB to 140 dB or even greater. Therefore, to avoid self-interference caused by a transmit signal of a transceiver to a receive signal of the transceiver, radio signal transmission and reception are generally differentiated by using different frequency bands or different time periods. For example, in a frequency division duplex (FDD) system, for transmission and reception, communication is performed by using different frequency bands separated by a guard band; in a time division duplex (TDD) system, for transmission and reception, communication is performed by using different time periods separated by a guard interval, where the guard band in the FDD system and the guard interval in the TDD system are both used to ensure that reception and transmission are fully isolated and to avoid interference caused by transmission to reception.
Different from the conventional FDD or TDD technology, a wireless full duplex technology may implement operations of reception and transmission simultaneously on a same radio channel. In this way, spectral efficiency of the wireless full duplex technology is theoretically twice that of the FDD or TDD technology. Apparently, a precondition for implementing wireless full duplex lies in that strong interference (referred to as self-interference, Self-interference) caused by a transmit signal of a transceiver to a receive signal of the transceiver is avoided, reduced, or canceled as much as possible, so that no adverse impact is caused to proper reception of a wanted signal.
FIG. 1 is a schematic block diagram of an interference suppression principle of a conventional wireless full duplex system. A digital-to-analog converter (DAC), an up converter, and a power amplifier in a transmit channel, and a low noise amplifier (LNA), a down converter, and an analog-to-digital converter (ADC) in a receive channel, and the like are functional modules of an intermediate radio frequency unit in a conventional transceiver. Canceling self-interference caused by a transmit signal is implemented by a spatial interference suppression unit, a radio frequency front-end analog interference cancellation module, a digital interference cancellation module, and the like that are shown in the figure.
Strength of a self-interference signal in a receive signal that undergoes spatial interference suppression is still far higher than that of a wanted signal, which causes blocking of front-end modules such as an LNA of a receiver. Therefore, before the LNA, the radio frequency front-end analog interference cancellation module uses a radio frequency signal coupled from a transmit-end power amplifier as a reference signal, and adjusts the reference signal by using estimated channel parameters such as an amplitude and a phase from a local transmit antenna to a local receive antenna, so that the reference signal approaches a self-interference signal component in a receive signal as much as possible. In this way, a local self-interference signal received by the receive antenna is canceled in an analog domain.
As shown in FIG. 1, in the conventional wireless full duplex system, radio frequency analog self-interference suppression is implemented before the LNA. In addition to a main-path self-interference signal component that is formed when the transmit signal arrives at the receive antenna after light-of-sight (LOS) propagation, the transmit signal also enters the receive antenna after being radiated by a scatterer during spatial propagation. Therefore, the self-interference signal further includes other components such as a near-field reflected self-interference signal and a far-field reflected self-interference signal.
FIG. 2 shows composition of a self-interference signal. As shown in FIG. 2, power of a far-field reflected self-interference signal component is very low, and does not have an adverse impact on a receive channel after an LNA, and therefore interference cancellation may be performed at a baseband by using a digital filter after an ADC. However, power of a near-field reflected self-interference signal component is relatively high, which may cause saturation of a receiver after the LNA.
Therefore, it is expected that a technology capable of canceling a near-field reflected self-interference component could be provided.